A circuit breaker is mainly composed of three basic parts, namely contacts, arc extinguishing system, and various release devices, including overcurrent release device, undervoltage release device, thermal release device, shunt release device, and free release device.

The circuit breaker switch is manually or electrically closed by the operating mechanism. After the contacts are closed, the free release mechanism locks the contacts in the closed position. When the above faults occur in the circuit, the free trip mechanism is activated by its respective release device to automatically trip and achieve protection. The shunt release is used for remote control of the circuit breaker.

The overcurrent release is used for short-circuit and overcurrent protection of the circuit. When the current of the circuit is greater than the set current value, the electromagnetic force generated by the overcurrent release causes the hook to trip, and the moving contact quickly disconnects under the tension of the spring, achieving the tripping function of the short-circuit device.

The thermal release is used for overload protection of the circuit, and its working principle is the same as that of a thermal relay.

The undervoltage release is used for undervoltage protection, as shown in Figure 1-8. The coil of the undervoltage release is directly connected to the power supply and is in a closed state. The circuit breaker can be closed normally; When there is a power outage or the voltage is very low, the suction force of the voltage loss release is less than the reaction force of the spring. The spring causes the moving iron core to move upward and release the hook, achieving the tripping function of the short-circuit device.

The shunt release is used for remote tripping. When the button is pressed remotely, the shunt release is energized and generates electromagnetic force, causing it to trip.

The protection of different circuit breakers is different, and should be selected according to the needs during use. The protection methods can also be marked in the graphic symbols, as shown in Figure 1-8. The circuit breaker graphic symbols indicate three types of protection methods: undervoltage, overload, and overcurrent.